The present invention relates to a printer, and more particularly to a structure of a printer so constructed as to permit a tractor as a printing sheet feed means to be attached selectively to an upstream side and a downstream side of a platen as viewed from the direction of feed of the printing sheet.
An arrangement in which two tractor units of an identical construction are disposed on an upstream side and a downstream side of a platen of a printer is known from Japanese Patent Publication (Kokoku) No. 60-36950. This arrangement needs a mechanism for enabling the downstream side tractor unit to be drivable alone. While the upstream side tractor unit is disconnected to a drive source, the downstream side tractor unit is driven to exert a tension on the printing sheet so that a tension is imparted on the printing sheet for operating the upstream side tractor with the tension. To this end, a clutch or the like mechanism is required, which mechanism, however, makes the printer complicated in construction and expensive to manufacture.
There has been provided a certain type of continuous-feed printing sheet such as a multiple-part printing sheet stack in which a plurality of sheets are stacked with one another for simultaneous printing. If printing is to be carried out with respect to such multiple sheet stack at one time, it is necessary to reduce contacting length of the multiple sheet stack with an outer peripheral surface of a platen in order to reduce relative displacement between superposing sheets (to maintain alignment among the multiple sheets), and to prevent the multiple sheet stack from being jamming in the vicinity of a supply opening reaching the platen.
To this end, a tractor unit is disposed on a downstream side of the platen as viewed from the direction of feed of the continuous-feed, multiple-part printing sheet stack. On the other hand, when a continuous-feed printing sheet composed of a single sheet is used, the printing sheet must be partly wound reliably around the platen to accomplish a high-speed printing. To this end, a tractor unit mounting portion is disposed on an upstream side of the platen as viewed from the printing sheet feed direction. This arrangement also has the advantageous of reducing the overall size of a printer.